MassMatrixAssembler.cpp

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#include "MassMatrixAssembler.hpp"
 
#include <polyfem/quadrature/TriQuadrature.hpp>
#include <polyfem/quadrature/TetQuadrature.hpp>
#include <polyfem/utils/MaybeParallelFor.hpp>
#include <polyfem/utils/GeometryUtils.hpp>
#include <polyfem/utils/ClipperUtils.hpp>
#include <polyfem/utils/Logger.hpp>
 
#include <SimpleBVH/BVH.hpp>
 
namespace polyfem
{
    using namespace basis;
    using namespace quadrature;
    using namespace utils;
 
    namespace assembler
    {
        namespace
        {
            class LocalThreadMatStorage
            {
            public:
                std::vector<Eigen::Triplet<double>> entries;
                StiffnessMatrix tmp_mat;
                StiffnessMatrix mass_mat;
                ElementAssemblyValues vals;
                QuadratureVector da;
                Quadrature quadrature;
 
                LocalThreadMatStorage()
                {
                }
 
                LocalThreadMatStorage(const int buffer_size, const int mat_size)
                {
                    init(buffer_size, mat_size);
                }
 
                void init(const int buffer_size, const int mat_size)
                {
                    entries.reserve(buffer_size);
                    tmp_mat.resize(mat_size, mat_size);
                    mass_mat.resize(mat_size, mat_size);
                }
 
                void condense()
                {
                    if (entries.size() >= 1e8)
                    {
                        tmp_mat.setFromTriplets(entries.begin(), entries.end());
                        mass_mat += tmp_mat;
                        mass_mat.makeCompressed();
 
                        tmp_mat.setZero();
                        tmp_mat.data().squeeze();
 
                        mass_mat.makeCompressed();
 
                        entries.clear();
                        logger().debug("cleaning memory...");
                    }
                }
            };
 
            // TODO: use existing PolyFEM code instead of hard coding these gmappings
            VectorNd P1_2D_gmapping(
                const Eigen::MatrixXd &nodes, const Eigen::Vector2d &uv)
            {
                assert(nodes.rows() == 3 && nodes.cols() == 2);
                return (1 - uv[0] - uv[1]) * nodes.row(0) + uv[0] * nodes.row(1) + uv[1] * nodes.row(2);
            }
 
            VectorNd P1_3D_gmapping(
                const Eigen::MatrixXd &nodes, const Eigen::Vector3d &uvw)
            {
                assert(nodes.rows() == 4 && nodes.cols() == 3);
                return (1 - uvw[0] - uvw[1] - uvw[2]) * nodes.row(0) + uvw[0] * nodes.row(1) + uvw[1] * nodes.row(2) + uvw[2] * nodes.row(3);
            }
        } // namespace
 
        void MassMatrixAssembler::assemble_cross(
            const bool is_volume,
            const int size,
            const int n_from_basis,
            const std::vector<basis::ElementBases> &from_bases,
            const std::vector<basis::ElementBases> &from_gbases,
            const int n_to_basis,
            const std::vector<basis::ElementBases> &to_bases,
            const std::vector<basis::ElementBases> &to_gbases,
            const AssemblyValsCache &cache,
            StiffnessMatrix &mass) const
        {
            const int buffer_size = std::min(long(1e8), long(std::max(n_from_basis, n_to_basis)) * size);
            // logger().debug("buffer_size {}", buffer_size);
 
            mass.resize(n_to_basis * size, n_from_basis * size);
            mass.setZero();
 
            // auto storage = create_thread_storage(LocalThreadMatStorage(buffer_size, mass.rows()));
 
            Quadrature quadrature;
            if (is_volume)
                TetQuadrature().get_quadrature(2, quadrature);
            else
                TriQuadrature().get_quadrature(2, quadrature);
 
            // Use a AABB tree to find all intersecting elements then loop over only those pairs
            std::vector<std::array<Eigen::Vector3d, 2>> boxes(from_bases.size());
            for (int i = 0; i < from_bases.size(); i++)
            {
                const Eigen::MatrixXd from_nodes = from_bases[i].nodes();
                boxes[i][0].setZero();
                boxes[i][0].head(size) = from_nodes.colwise().minCoeff();
                boxes[i][1].setZero();
                boxes[i][1].head(size) = from_nodes.colwise().maxCoeff();
            }
 
            SimpleBVH::BVH bvh;
            bvh.init(boxes);
 
            // maybe_parallel_for(n_to_basis, [&](int start, int end, int thread_id) {
            // LocalThreadMatStorage &local_storage = get_local_thread_storage(storage, thread_id);
 
            std::vector<Eigen::Triplet<double>> triplets;
 
            for (const ElementBases &to_element : to_bases)
            {
                const Eigen::MatrixXd to_nodes = to_element.nodes();
 
                std::vector<unsigned int> candidates;
                {
                    Eigen::Vector3d bbox_min = Eigen::Vector3d::Zero();
                    bbox_min.head(size) = to_nodes.colwise().minCoeff();
                    Eigen::Vector3d bbox_max = Eigen::Vector3d::Zero();
                    bbox_max.head(size) = to_nodes.colwise().maxCoeff();
                    bvh.intersect_box(bbox_min, bbox_max, candidates);
                }
 
                // for (const ElementBases &from_element : from_bases)
                for (const unsigned int from_element_i : candidates)
                {
                    const ElementBases &from_element = from_bases[from_element_i];
                    const Eigen::MatrixXd from_nodes = from_element.nodes();
 
                    // Compute the overlap between the two elements as a list of simplices.
                    const std::vector<Eigen::MatrixXd> overlap =
                        is_volume
                            ? TetrahedronClipping::clip(to_nodes, from_nodes)
                            : TriangleClipping::clip(to_nodes, from_nodes);
 
                    for (const Eigen::MatrixXd &simplex : overlap)
                    {
                        const double volume = abs(is_volume ? tetrahedron_volume(simplex) : triangle_area(simplex));
                        if (abs(volume) == 0.0)
                            continue;
                        assert(volume > 0);
 
                        for (int qi = 0; qi < quadrature.size(); qi++)
                        {
                            // NOTE: the 2/6 is neccesary here because the mass matrix assembly use the
                            //       determinant of the Jacobian (i.e., area of the parallelogram/volume of the hexahedron)
                            const double w = (is_volume ? 6 : 2) * volume * quadrature.weights[qi];
                            const VectorNd q = quadrature.points.row(qi);
 
                            const VectorNd p = is_volume ? P1_3D_gmapping(simplex, q) : P1_2D_gmapping(simplex, q);
 
                            // NOTE: Row vector because evaluate_bases expects a rows of a matrix.
                            const RowVectorNd from_bc = barycentric_coordinates(p, from_nodes).tail(size).transpose();
                            const RowVectorNd to_bc = barycentric_coordinates(p, to_nodes).tail(size).transpose();
 
                            std::vector<AssemblyValues> from_phi, to_phi;
                            from_element.evaluate_bases(from_bc, from_phi);
                            to_element.evaluate_bases(to_bc, to_phi);
 
#ifndef NDEBUG
                            Eigen::MatrixXd debug;
                            from_element.eval_geom_mapping(from_bc, debug);
                            assert((debug.transpose() - p).norm() < 1e-12);
                            to_element.eval_geom_mapping(to_bc, debug);
                            assert((debug.transpose() - p).norm() < 1e-12);
#endif
 
                            for (int n = 0; n < size; ++n)
                            {
                                // local matrix is diagonal
                                const int m = n;
                                {
                                    for (int to_local_i = 0; to_local_i < to_phi.size(); ++to_local_i)
                                    {
                                        const int to_global_i = to_element.bases[to_local_i].global()[0].index * size + m;
                                        for (int from_local_i = 0; from_local_i < from_phi.size(); ++from_local_i)
                                        {
                                            const auto from_global_i = from_element.bases[from_local_i].global()[0].index * size + n;
                                            triplets.emplace_back(
                                                to_global_i, from_global_i,
                                                w * from_phi[from_local_i].val(0) * to_phi[to_local_i].val(0));
                                        }
                                    }
                                }
                            }
                        }
                    }
                }
            }
 
            mass.setFromTriplets(triplets.begin(), triplets.end());
            mass.makeCompressed();
        }
 
    } // namespace assembler
} // namespace polyfem